Light into which information is encoded enables humanly imperceptible transmission of data or information using otherwise visible light emitted by a light source of an illumination system. Particularly in the context of installation and maintenance of such illumination systems, the data or information may comprise a light source identifier, but also light source status information such as temperature, accumulated operating hours, etc.
One technique that has been proposed to provide transmission of such data or information is based on embedding codes, or ‘identifiers’, identifying a light source or a group of light sources by modulating the light output thereof in a manner so that the embedded codes are invisible to the users. Such light output is sometimes referred to as ‘coded light’.
Detection of the data or information typically takes place by using a photodiode as a light sensor. In a situation where light originating from several light sources is detected by the photodiode, the possibility to separate the data transmissions of the respective light sources depends on the chosen transmission protocol. The transmission protocol is essentially a network protocol that functions in a similar way as in a data network. For practical reasons, the communication typically takes place in an asynchronous fashion, by transmitting data in separate short binary sequences called packets. To enable detection of several light sources using a single photodiode the so-called ALOHA protocol is used (see N. Abrahamson, “The ALOHA system—Another alternative for computer communications”, in Proc. Fall Joint Computer Conference, AFIPS Press, pp. 281-285 [1970]). According to the ALOHA protocol, each repeated packet transmission takes place after a random time interval. As a result, the total average transmission time per packet is equal to the sum of the packet duration and the average random time interval. In case there are additional light sources in the field of view of the photodiode, simultaneously transmitted packets collide and are therefore lost.
It has been demonstrated that the average transmission time required for successful reception of all light sources in the field of view of the photodiode increases exponentially with the number of light sources present within the field of view of the photodiode. For detection of data encoded into light from several light sources, in case rapid detection of the data is not required, the exponential increase of the required average transmission time, i.e. the required time for performing the detection of data from all light sources, may not constitute a problem. However, there are situations in which a protocol-based approach for detection of data encoded into light from several light sources may not be appropriate. One of such situations is when the available time for performing the detection is limited, for instance, in the case of detection of light emitted from several light sources from a position in a moving vehicle. Another one of such situations is where there is a relatively large number of light sources transmitting information. Such a situation can for example occur when the detection takes place from a position that is far away from the light sources, e.g., detection of light sources on the ground from a position in a helicopter in the air. A photodiode used for detecting light originating from several light sources has a limited directional resolution. Thus, in the latter situation where the distance between the light sources and the photodiode is large, it will become difficult to aim the photodiode at individual light sources.